Sequential two-stage displacive transformation from β to α via β′ phase in polymorphic MnTe film

MnTe films exhibit a remarkable change in physical properties upon polymorphic transformation from β (wurtzite-type) to α (nickeline-type) phase. In this study, the mechanism of the β to α transformation in MnTe films capped with a tungsten layer during isothermal annealing at 500 °C was investigated. X-ray diffraction analyses revealed that the out-of-plane c-axis orientation was maintained during the polymorphic transformation. High-resolution transmission electron microscopy revealed that the β-phase transformed to the α-phase via a β′-phase which has a wurtzite-type structure as the β-phase but with a slight difference in the coordinates of the Te atoms. The β → β′ transformation was induced by the puckering process during which Mn- and Te-atomic planes shift in opposite directions along the c-axis, whereas the β′ → α transformation was induced by a buckling process during which pairs of Mn- and Te-atomic planes alternately move to the opposite directions along the 210 direction. Meanwhile, in the MnTe film without the tungsten capping layer, the out-of-plane c-axis orientation was not retained during the polymorphic transformation. These results imply that the sequential two-stage polymorphic transformation maintaining the out-of-plane c-axis orientation is caused by the constraint on the MnTe film induced by the tungsten capping layer. [Display omitted] •The β-MnTe film was found to transform to the α-MnTe via a β′-MnTe as an intermediate state.•The β-MnTe film with a W cap layer showed the transformation while maintaining the out-of-plane c-axis orientation.•The β-to-β′ and β′-to-α transformations were induced by the puckering and buckling processes, respectively.•The sequential two-stage polymorphic transformation is caused by the mechanical constraint between the MnTe and W layer.